Time and space distribution of the Neogene intrusive magmatism from Oaş-Gutâi Mts., Eastern Carpathians, Romania

The Oaş-Gutâi Mts. (OG) belongs to the Eastern Carpathians Neogene-Quaternary volcanic chain. Two types of calc-alkaline volcanism took place during the Middle-Miocene (15.4-7.0 Ma): a felsic explosive extensional type and an intermediate arc type of extrusive and intrusive origin, respectively. Gold-silver and base metal epithermal ore deposits are associated with the intermediate type of volcanism.
   Subvolcanic and shallow–level intravolcanic intrusive rocks of irregular shapes and various sizes (from tens of metres up to 6 km long) developed on more of 3000 m vertical extent (based on drill core data). They suggest morphologies of dykes, sills and apophysis of microlaccolites crosscutting the Paleogene flysch-type basement, the Neogene sedimentary deposits, as well as the volcanic suite; hundreds of intrusions outcrop mostly in the southeastern part of Gutâi Mts. Despite the relationships of the intrusions with different volcanic complexes, they can be hardly attributed to some individual volcanic structures. Among the various compositions and textures of rock types (from gabbros to microgranodiorites), the andesites and the porphyritic microdiorites, quartz diorites and quartz monzodiorites are the most abundant. Hornfelses and sometimes skarns formed at the contact of the intrusions with the sedimentary deposits.
   The spatial distribution of the intrusions in the Gutâi Mts. suggests a possible connection with the major transcrustal fault/ tectonic system Bogdan Vodă-Dragoş Vodă, developed in the southern part of the volcanic area, as well as with some tectonic alignments showing the same orientation as most of the hydrothermal veins. The inception of the intrusive phases can be related to the change of the regional tectonic regime from transpressional to transtensional at 12 Ma, as it was recently invoked in the case of the “Subvolcanic Zone” of the Eastern Carpathians. It ended at 9.2 Ma, except for the late mafic/basaltic phase from Gutâi Mts. (8.1-7.0 Ma) ceasing the magmatism. In Oaş Mts. the intrusions were emplaced exclusively in Pannonian (10.8-9.6 Ma), while in Gutâi Mts. the intrusive magmatism started in Sarmatian (11.9-11.4 Ma) postdating the Sarmatian volcanism (13.4-12.1 Ma). Along with the emplacement of the Pannonian volcanics, intrusive rocks with different K-Ar ages (11.7-9.2 Ma) were also emplaced, being attributed to different volcanic phases. The main intrusive magmatism occurred contemporaneously with the paroxysm of the OG volcanism. The time intervals of the intrusion emplacement in OG and Poiana Botizei and Ţibles from the “Subvolcanic Zone”, respectively are similar except for the mafic intrusive phase from OG.
   Important veins are hosted by the intrusions from OG. Comparing the age intervals of the mineralisations with those of the intrusions, seems that the radiometric data achieved on adularia and illite from the epithermal ore deposits are consistent with those of the intrusions identified allover the metallogenetic fields: e.g. the Ilba-Nistru base metal metallogenetic field from the south-western part of Gutai Mts. shows 11.9-11.4 Ma interval of K-Ar ages for the intrusive rocks, 11.6-10.7 Ma interval for the K-Ar ages of adularia and illite and 10.6 Ma Ar-Ar age for adularia, exclusively. Except for the basaltic intrusive complex, the intrusions from OG are older than the epithermal mineralisations from all the ore deposits.
   Acknowledgements: This work was supported by OTKA grant No. K68153.